I. Field of the Invention
The present invention relates to long term storage of gases under pressure and more specifically to long term storage of carbon dioxide under pressure so that it is available for use in a fire suppression system.
II. Description of the Prior Art
In a typical fire suppression system, carbon dioxide (CO.sub.2), is maintained or stored in one or more pressure vessels (i.e., tanks or canisters). The pressure vessels are connected through a valve to a piping system for releasing the CO.sub.2 in the area of a fire. As will be appreciated, it may be necessary to store the CO.sub.2 for long periods of time in order to ensure availability of carbon dioxide in the event of a fire.
Two types of carbon dioxide storage systems have typically been employed for fire suppression systems. These two systems may be referred to as high pressure systems (e.g., about 850 psi) and low pressure systems (e.g., about 300 psi), respectively. Each type of system has provided much needed long-term storage of carbon dioxide, but not without some significant drawbacks.
Low pressure systems have typically been employed for storing extremely large quantities of carbon dioxide in excess of 1000 lbs. such as up to several tons. In order to prevent loss of carbon dioxide which could occur as the carbon dioxide warms up, low pressure systems typically also refrigerate the storage tanks. By refrigerating the tank, the carbon dioxide is kept in a liquid state such as at around 0.degree. F. and thus is less likely to boil off. But maintaining the tank at such a cold temperature has conventionally required very large mechanical compressor-based refrigeration systems.
Compressor-based systems not only require substantial space, but they are very heavy, require periodic servicing, and utilize refrigerants, such as CFC's, which are known to be environmentally undesirable. And should the compressor system fail, lose power, or leak, not only might hazardous refrigerants be expelled into the environment, but the liquid carbon dioxide will begin to heat up and go into its vapor state where it might then boil off from the tank resulting in loss of fire suppression capability.
In those situations where lesser quantities of carbon dioxide are necessary (such as less than 1000 lbs.), high pressure systems are preferred. High pressure systems eliminate the refrigerator and its drawbacks, but at the expense of introducing a different set of problems. In high pressure systems, each pressure vessel is typically designed to hold no more than about 100 lbs. of carbon dioxide. Consequently, to provide sufficient carbon dioxide capacity to suppress fires, it is typical to connect several such pressure vessels together such as through a manifold. The complexity of multiple vessel systems and the space requirements imposed by adding tanks limits the utility of such high pressure systems to typically low capacity situations.
Further, because the carbon dioxide is stored under high pressure, it is not typical to refrigerate the tanks. Thus, refrigerators employed in larger systems are not necessary thereby eliminating the drawbacks associated therewith. But one result of not refrigerating the high pressure tanks is that, over time, carbon dioxide may boil off. To avoid losing so much of the CO.sub.2 that the fire suppression system becomes ineffective or useless, periodic testing of the high pressure vessels becomes necessary.
Testing of the high pressure vessels typically requires that each tank be individually removed from the system and weighed. If the weight of the pressure vessel is too low (indicating loss of CO.sub.2), then the tank must be recharged with more carbon dioxide. The tested tank must then be reconnected to the system. These tasks are not only time consuming and introduce human error, but if not done in a timely fashion could lead to a failure of the fire suppression system for lack of sufficient carbon dioxide.
To avoid CO.sub.2 boiling off in the high pressure systems, it might be possible to refrigerate the tanks as done in low pressure systems. However, size considerations alone, not to mention weight and other problems of compressor-based refrigerators, militate against their use where only low quantities of CO.sub.2 (less than 1000 lbs.) are needed for the fire suppression system.